The present invention relates to a method of separating oxygen from an oxygen containing gas with the use of composite ceramic membranes. More particularly, the present invention relates to such a method in which the composite ceramic membrane has multiple layers comprising a dense layer, one or more active porous layers, and one or more porous support layers. Even more particularly, the present invention relates to such a method in which the pore radii, the distribution of pore radii, and thickness of the active porous layer are selected for optimum transport through the membrane.
The development of ceramic materials that have high oxygen ion conductivity at elevated temperatures of ranging from between about 600xc2x0 C., and about 1100xc2x0 C. has created interest in employing membranes fabricated from these materials in separating oxygen from an oxygen containing gas stream. If the material or membrane also has electronic conductivity, electrons are returned within the membrane and the driving force for ion transfer is provided by a positive ratio of oxygen partial pressures at the feed or cathode side and the permeate or anode side respectively. Dense film membranes require reasonably thick (1 to 2 mm) membranes for structural integrity and, since the oxygen flux is inversely proportional to the membrane thickness, such membranes achieve only moderate oxygen flux levels. Providing a thin active membrane layer supported by a porous matrix, as in a composite membrane, minimizes the resistance of the dense layer. However, other types of resistance such as oxygen surface exchange and chemical potential drop in the support matrix become predominant limitations in such composite ceramic membranes. Composite ceramic membranes, in which one or several porous layers support a thin dense membrane film and in which the limitations mentioned above are addressed, provide significant opportunities for increasing the oxygen flux across the membrane over the levels that are able to be achieved by unsupported dense membranes.
The prior art suggests that there exists a dependency between oxygen flux and the pore size of the pores within the porous layers. For instance, T. Kenjo et al., xe2x80x9cLaMnO3 Air Cathodes Containing ZrO2 Electrolyte for High Temperature Oxide Fuel Cellsxe2x80x9d, Solid State Ionics. 57, 1992, pgs. 259-302 discloses that enhancement of oxygen flux across a thin membrane requires an active electron and ion conducting layer adjacent to the dense electrolyte and suggests that flux may be enhanced by decreasing pore diameter. With respect to porous layer thickness it is further shown that there is a limiting thickness beyond which no further enhancement is realized. Similarly, Deng et al., xe2x80x9cTransport in solid oxide porous electrodes: Effect of gas diffusionxe2x80x9d, Solid State Ionics 80, 1995, pgs. 213-222, recognizes an optimum thickness of a porous layer for a maximum level of enhancement of oxygen flux and also, that the location of the optimum varies with the surface area of the pores per unit volume of the layer. U.S. Pat. No. 5,240,480 describes a composite membrane in which a plurality of porous layers support a dense layer. The pore radii of all of the porous layers are less than 10 microns and the pore radii of successive porous layers increase in a direction taken from the dense layer. The porous layer contiguous to the dense layer possesses both ionic and electronic conductivity and the pore radius of the active layer is sufficiently small so as to achieve a significant enhancement of gas phase to electrolyte surface exchange.
None of the prior art discusses the fact that real porous surfaces have not a single pore radius but a distribution of pore radii and that this distribution affects the level (relative to an optimum flux) that can be attained for a given thickness of the porous layer. As will be discussed, the inventors have found that transport through a composite membrane may be optimized beyond that thought possible in the prior art through a relationship of the thickness of the active porous layers to the pore radius and the definition of a permissible range in the standard deviation for the pore radius distribution from the optimum radius.
The present invention provides a method of separating oxygen from an oxygen containing gas with a composite membrane capable of conducting oxygen ions and electrons. In accordance with the method, the composite membrane is subjected to an operational temperature and the oxygen containing gas with a higher oxygen partial pressure at a cathode side thereof and a lower partial oxygen pressure established at an anode side thereof. It is to be noted that the operational temperature of such a membrane is well known and as indicated above, is typically within a range of between about 600xc2x0 C. and about 1100xc2x0 C. Lower and higher operational temperatures have been considered within the prior art. The oxygen containing gas can be air or a gas having oxygen in a bound state, for instance water. Further, the oxygen so separated from the oxygen containing gas can be recovered or further reacted to produce a product at the other side of the membrane, known as the anode side.
The composite membrane has a dense layer, at least one active porous layer contiguous to the dense layer, and at least one porous support layer. The term xe2x80x9cactivexe2x80x9d as used in connection with a porous layer means a layer that is comprised of materials that have both oxygen ion and electron conducting conductivity and include both mixed conducting metallic oxides and multi phase mixtures of oxygen ion conducting metallic oxides and electron conducting oxides and/or electron conducting metals.
The active porous layer has a thickness and a distribution of pore radii. To obtain an oxygen flux at more than 80 percent of the maximum oxygen flux for that layer thickness, the standard deviation of the distribution of the log of the pore radii is equal to a product of 1.45 and a shape factor, the shape factor being greater than 0 and no greater than about 0.5. The thickness is about equal to the product of a constant and the area weighted average of the pore radii. The constant is a function of a material used to fabricate the active porous layer, the operational temperature, an oxygen partial pressure within the active porous layer, and a porosity and a tortuosity produced by the arrangement of pores in the porous layer.
Specifically, the constant for determining the optimum thickness in meters is determined from the relationship:             T              94      ,      695        ⁢      (          ⅇ                                                  E              σ                        -                          E              k                                16.63                ⁢                  (                                    1              T                        -                          1              1273                                )                      )    ⁢                              σ          ion                ⁡                  (                      1            -            ϵ                    )                            k        ⁢                  xe2x80x83                ⁢                  P                      O            2                    0.5                ⁢        ϵ        ⁢                  xe2x80x83                ⁢        τ            
where:
"sgr"ion is an ionic conductivity of the active porous layer [Ohmxe2x88x921/m]
T is the operational temperature [K]
k is a surface exchange factor in the porous layer at 1273 K [mol O2/m2s barn]
PO2 is an oxygen partial pressure at said active porous layer [bar]
xcex5 is a porosity of the active porous layer
E"sgr" is the activation energy for ion conductivity [J/mol]
Ek is the activation energy for surface exchange [J/mol]
xcfx84 is the tortuosity of the active porous layer.
Where thickness and radius are related by this constant, a maximum oxygen transport or flux can be effected through such a porous layer for a given set of operating conditions. As stated previously, when a porous membrane is fabricated, the pore size varies. The inventors have found, quite unexpectedly, that it is important to construct the membrane with a very specific pore size distribution to obtain an optimum membrane in terms of oxygen transport through the membrane.
The permissible standard deviation of the logarithm to the base 10 of the pore radii that yields more than 80 percent, and preferably more than 90 percent, of the maximum flux is the product of 1.45 times a shape factor that for 80 percent is greater than 0 and no greater than 0.5, and preferably no greater than 0.4 for a flux at 90 percent. Practically, the distribution of pore radii can be determined by optical means, mercury porosimetry, and gas adsorption measurements. The logarithm of each radius in the distribution is then determined from such a result and the standard deviation of the logarithm can be obtained.
As may be appreciated it becomes increasingly more difficult, if not prohibitably expensive, to construct a membrane having pore radii with no deviation. Pore radii that yield a membrane with about an 80 percent flux capability is minimally acceptable. At the other end of the spectrum, 98 percent above is possible if the shape factor is limited to be no greater than about 0.2 and the thickness of the membrane is about equal to a product of the constant and the square root of an area weighted average of the pore radii. An xe2x80x9carea weighted average of the pore radiixe2x80x9d as used herein and in the claims means a distribution of radii; ravg, which is obtained from the distribution of radii by:       r    avg    =                    ∑        i            ⁢                        r          i                ⁢                              A            i                    /                      V            i                                              ∑        i            ⁢                        A          i                /                  V          i                    
Where:
ravg is the area per unit volume weighted radius
ri is the radius of individual pores and varies from the smallest to the largest radius in the distribution with radii smaller than 0.01 xcexcm not counted for operating temperatures smaller than 850xc2x0 C. and radii smaller than 0.1 xcexcm not counted for operating temperatures greater than 850xc2x0 C.
Ai is the sum of the surface areas of all pores of radius ri 
Vi is the sum of the volumes of all pores of radius i
In the present invention it has been found, unexpectedly, that to obtain optimum performance, the thickness of the active porous layer must decrease in a predetermined relationship with the decrease in radius where the effective pore radii are between 0.01 and 5 microns. Pores with radii smaller than about 0.01 microns will close due to sintering at high temperature operational conditions for the membrane. The value is material and operating temperature dependent. The lower value of 0.01 is applicable at lower membrane operating temperatures of no greater than about 850xc2x0 C. Pores with radii 0.1 and less will be closed at temperatures of between about 850xc2x0 C. and about 1100xc2x0 C. A minimum pore radius of about 0.2 is preferred for operational temperatures at and above about 1000xc2x0 C. Pore radii larger than about 5 microns provide insufficient surface exchange area.
The relationship for maximum flux can be expressed as a ratio of thickness and the area weighted average of the pore radii set equal to the constant divided by the square root of the area weighted average of the pore radii. The effective interval of the layer thickness over pore radii ratios associated with the above range was found to be between about 3 and 3000 with the lower value associated with the larger radii and the higher value with the smaller radii. Ratios lying in a range of between about 10 and about 2000 are preferred for tortuosities between about 1.2 and about 2.5. For tortuosities of between about 2.3 and 5.0, ratios of between about 6 and about 300 are preferred. For a tortuosity lying between about 5.0 and about 10.0, ratios from between about 4 and about 200 are preferred.
The relationship of thickness to radius is applicable for porous layers on the retentate or cathode side with gas mixtures comprising oxygen and nitrogen and on the anode or permeate side with the use of purge or sweep gases such as steam, carbon dioxide and nitrogen. For an anode side porous layer seeing only oxygen, the optimum thickness to pore radius ratio will be larger. However the optimum will be very flat and diminishing returns reached at a level calculated from the above equation.
The range of effective and practical porosities for membranes with the above parameters is between about 20 and about 60 percent, preferably no less than about 35 percent, and suitable tortuosities are between 1.0 and 5.0. It should be noted that the constant in the pore radius relationship contains (i) the morphological properties of the porous layer, such as porosity and tortuosity, (ii) the physical properties of the layer material, such as ion conductivity and activation energy for ionic conductivity as well as the surface exchange factor and the activation energy for surface exchange, and (iii) temperature and oxygen partial pressure which define the operating environment. Thereby corrections are provided to adjust the optimum ratio of thickness over pore ratio to fit specific selections. The relationship indicates a partial oxygen pressure and operating temperature dependence of the optimum value for t/r. In applications involving the separation of oxygen from air, the pressure dependence has an impact since the partial oxygen pressure can decrease by a factor of more than 4 or more on the cathode as the retentate stream is depleted in oxygen; or on the anode when use of a purge gas leads to a significant variation in oxygen partial pressures on the permeate side or anode. In these cases increasing the thickness of the active porous layers continuously or stepwise, in accord with the equation, as the partial oxygen pressure decreases will lead to higher oxygen fluxes. Similarly, in applications involving reactions of oxygen with a fuel species on the anode, variations in local membrane temperature can be expected. Again the thickness of the active layers adjacent to the dense membrane layer can be adjusted to obtain optimum oxygen flux.
The reason for limiting the thickness of the active layer at smaller pore radii is that at smaller pore radii more chemical potential is consumed per unit thickness to the extent that the flux will actually decrease when the limiting value is exceeded. High tortuosity will increase the drop in chemical potential in the gas phase as well as in the solid phase and therefore reduce the limiting value for the thickness and the ratio of thickness over pore radius. The invention also indicates that it is advantageous to make the pore radii as small as feasible from manufacturing and survivability considerations.
The parameter ranges specified above produce the conditions required to construct active layers that are both efficient and practical. The invention in a further aspect provides guidance for the construction of the inactive support layers, which should consume as little pressure drop as possible. In this regard, the pores of the porous support layers should be larger than those of an adjacent active porous support layer. The at least one porous support layer is commonly contiguous with the at least one active porous layer. It is advantageous to employ multiple support layers with the pore radii increasing away from the active layer. Preferably there are between 1 and 5 of such porous support layers. Pore radii and porosity of each support layer should be as large as possible with the constraint that selected values must still be small enough to support the adjacent layer during manufacturing and operation and maintain mechanical integrity. To meet this requirement, it is desirable to select the pore radius ratio of adjacent layers to be between about 2 and about 15 and preferably between about 5 and about 10 and minimum thickness of these layers at above about 10 times the average pore radius.
The porosity of the porous support layers is preferably greater than about 35 percent. In case multiple layers are used the porous support layers and the active porous layer can be fabricated from materials having different coefficients of thermal expansion with the coefficients of thermal expansion of the porous support layers situated between the outermost of the porous support layers and the dense layers have magnitudes between those of the outermost support layer and the active porous layer. The materials of the porous support layers situated between the outermost of said porous support layers and the dense layer preferably contain a mixture of those materials used in fabricating the active porous layer and the outermost support layer with the content of the active porous layer material decreasing away from that layer.
Two of the at least one active porous layers can be provided to sandwich the dense layer. In this case each of the two layers can be optimized independently. One or more porous support layers can be located on the anode or permeate side of the composite membrane.
It is to be noted that the present invention has applicability that is not limited to the separation and recovery of oxygen. The method of the present invention can be applied to the production of syngas and to such end, a reforming catalyst can be deposited on the surface of the pores of the at least one porous support layer. Alternatively or in addition, a reforming catalyst is located adjacent to or proximate the at least one porous support. The method of the present invention also has applicability to the introduction of a fuel to the anode side of the membrane and reacting the fuel with the permeated oxygen.